This invention relates to admixture compositions for incorporation into hydraulic cement mixes, such as concretes, mortars and grouts and dry mixes for making these, to provide economy, compressive strength benefits at all ages, and a desirable degree of acceleration of rate of hardening and setting.
The use of chemical admixtures to achieve these effects is well known in the art. For example, calcium chloride is well known as an economic and effective accelerator, but has the disadvantage of promoting corrosion of metal embedments at certain dosages, as well as other disadvantages. Thus, there is a need for economical and effective agents to replace calcium chloride in this application, and this has promoted a continuing search on the part of the industries involved.
Thus, U.S. Pat. No. 3,782,991 issued to Burge discloses anhydrous aluminum sulfate to increase early compressive strength of building materials, together with triethanolamine as a set accelerator.
Also, U.S. Pat. No. 4,298,392 issued to Isselmann discloses an admixture comprised of a combination of silica sol and sodium thiocyanate employed to increase compressive strength of cementitious systems prior to 72 hours. However, his data show that the desired effect is not achieved by the thiocyanate alone, but only by the combination.
In addition, U.S. Pat. No. 4,256,500 issued to Turpin discloses a variety of inorganic compounds for use in cementitious systems containing pozzolanic materials in order to obtain cement compositions more economical than compositions of equivalent compressive strength that previously have been employed. The chemical agents employed alone or in combination include alkali thiosulfate, sulfate, sulfite, nitrate, nitrite, thiocyanate, cyanide, chlorides and others. Superior effects were secured by combinations of sodium thiocyanate and sodium thiosulfate as shown in Turpin's Table 12, but even more complex mixtures yielded good results as shown in his Table 13 for sodium nitrate plus sodium thiosulfate, plus sodium sulfite plus sodium sulfate.
There is also U.S. Pat. No. 4,337,094 issued to Tokar disclosing as an accelerator for cementitious systems and also to improve compressive strength, a combination of calcium nitrate in the range of 0.5 to 3.0% based on weight of cement and an alkanolamine, with or without minor amounts of gluconates or lignosulfonates.
U.S. Pat. No. 4,373,956 issued to Rosskopf discloses an admixture consisting of an alkali or alkaline earth or ammonium salt of thiocyanic acid in combination with an alkanolamine which when added to a cementitous mix will increase the rate of hardening of the mix and will increase the compressive strength of the mix after hardening.
The art just cited approaches the goal of securing a desired degree of acceleration of rate of set of cementitious systems without sacrifice to either early or ultimate compressive strength, but still greater benefits of this type are desired than are possible with use of the admixtures thus far developed. Therefore, it would be highly advantageous to develop an admixture which would accelerate the rate of hardening and increase compressive strength of the hardened mix to a still greater degree, and to do so in an economical manner.